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    <script>
        /* && || ! */
        /* 
        && 逻辑与 两边都为true  结果才为true
        || 逻辑或 只要有一边为true结果就为true
        !   取反
         */
         var a=20;
         var b=30;
         var c=40;
         var d=50;
         console.log(a<b && c<d)  //true
         console.log(a<b && c>d)  //false
         console.log(a<b || c>d)  //true
         console.log(!a)  //false
    </script>
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